Field of the Invention
The present invention relates to a screw vacuum pump and, more particularly, to a screw vacuum pump which is designed so that it is possible to reduce the load on the pump at the time of starting and evacuation of a gas under atmospheric pressure.
There has heretofore been one type of screw vacuum pump which has a pair of male and female rotors rotating in mesh with each other around two parallel axes, respectively, and a casing for accommodating the two rotors, the casing having a suction port and a discharge port. The operation of the screw vacuum pump comprises a process of sucking a gas from the suction port into a space defined between the rotors, a process of compressing the gas inside the rotors, and a process of discharging the gas from the discharge port.
An advantageous way of obtaining a high degree of vacuum in the screw vacuum pump having the above-described arrangement is to increase the built-in volume ratio, that is, the compression ratio. However, in such a case, excessive power is needed at the time of starting and when a gas of atmospheric pressure is evacuated from the chamber during the top-speed operation. The following measures have heretofore been used in order to cope with the above-described problem:
(1) A method wherein a throat is attached to the suction pipe to lower the pressure of the gas sucked into the pump.
(2) A method wherein a gas relief mechanism is provided for groove spaces where high pressure is produced.
(3) A method wherein the rotating speed is lowered by using an inverter or the like.
(4) A method wherein a motor of large capacity is used.
The conventional methods (1) to (4) suffer, however, from the following disadvantages: Method (1) causes a lowering in the pumping speed and hence takes much time to evacuate the chamber. Method (2) leads to an increase in cost and lacks reliability. Method (3) leads to an increase in cost because of the need for an inverter or the like to change the rotating speed. Method (4) lacks compactness and leads to an increase in cost because of the use of a motor of large capacity.